This application incorporates by reference the disclosure of U.S. Provisional Patent Application Ser. No. 60/470,026 filed May 12, 2003 and entitled “RF MEMS Switch with Integrated Impedance Matching Structure”.
In one aspect, this invention addresses several problems with existing single-pole, multi-throw switches built using single-pole, single-throw devices preferably combined in a switch matrix. According to this aspect of the invention, the switches are symmetrically located around a central point which is preferably a vertical via in a multi-layer printed circuit board. In this way, a maximum number of switches can be located around the common port with a minimum amount of separation. This leads to the lowest possible parasitic reactance, and gives the circuit the greatest possible frequency response. Furthermore, any residual parasitic reactance can be matched by a single element on the common port, so that all ports will have the same frequency response. This patent describes a 1×4 switch, but the concept may be extended to a 1×6 switch or to a 1×8 switch or a switch with even greater fan out (1×N). Also, such a switch can be integrated with an antenna array for the purpose of producing a switched beam diversity antenna.
The switch arrangement disclosed herein can be conveniently used with a Vivaldi Cloverleaf Antenna to determine which antenna of the Vivaldi Cloverleaf Antenna is active. U.S. patent application Ser. No. 09/525,832 entitled “Vivaldi Cloverleaf Antenna” filed Mar. 12, 2000, the disclosure of which is hereby incorporated herein by this reference, teaches how Vivaldi Cloverleaf Antennas may be made.
The present invention has a number of possible applications and uses. As a basic building block in any communication system, and in microwave systems in general, a single-pole, multi-throw radio frequency switch has numerous applications. As communication systems get increasingly complicated, and they require diversity antennas, reconfigurable receivers, and space time processing, the need for more sophisticated radio frequency components will grow. These advanced communications systems will need single-pole multi-throw switches having low parasitic reactance. Such switches will be used, for example, in connection with the antenna systems of these communication systems.
The prior art includes the following:                (1) M. Ando, “Polyhedral Shaped Redundant Coaxial Switch”, U.S. Pat. No. 6,252,473 issued Jun. 26, 2001 and assigned to Hughes Electronics Corporation. This patent describes a waveguide switch using bulk mechanical actuators.        (2) B. Mayer, “Microwave Switch with Grooves for Isolation of the Passages”, U.S. Pat. No. 6,218,912 issued Apr. 17, 2001 and assigned to Robert Bosch GmbH. This patent describes a waveguide switch with a mechanical rotor structure.        
Neither of the patents noted above address issues that are particular to the needs of a single-pole multi-throw switch of the type disclosed herein. Although they are of a radial design, they are built using a conventional waveguide rather than (i) MEM devices and (ii) microstrips. It is not obvious that a radial design could be used for a MEM device switch and/or a microstrip switch because the necessary vertical through-ground vias are not commonly used in microstrip circuits. Furthermore, the numerous examples of microstrip switches available in the commercial marketplace do not directly apply to this invention because they typically use PIN diodes or FET switches, which carry certain requirements for the biasing circuit that dictate the geometry and which are not convenient for use in a radial design.
There is a need for single-pole, multi-throw switches as a general building block for radio frequency communication systems. One means of providing such devices that have the performance required for modern Radio Frequency (RF) systems is to use RF Micro Electro-Mechanical System (MEMS) switches. One solution to this problem would be to simply build a 1×N monolithic MEMS switch on a single substrate. However, there may be situations in which this is not possible, or when one cannot achieve the required characteristics in a monolithic solution, such as a large fan-out number for example. In these situations, a hybrid approach should be used.
There are numerous ways to assemble single-pole, single-throw RF MEMS switches on a microwave substrate, along with RF lines to create the desired switching circuit. Possibly the most convenient way is shown in FIG. 1. A common port, represented here as a microstrip line 5, ends at a point 6 near which several RF MEMS switches 10-1 through 10-4 are clustered. RF MEMS switches 10-1 through 10-4 are preferably spaced equidistant from a centerline of microstrip 5 and laterally on each side of it. Ports 1, 2, 3, and 4 then spread out from this central point 6, with each port being addressed by a single MEMS switch 10. The substrate, of which only a portion is shown, is represented by element 12. By closing one of the switches (for example, switch 10-4), and opening all of the others (for example, switches 10-1 through 10-3), RF energy can be directed from the common port provided by microstrip line 5 to the chosen selectable port (port 4 in this example) with very low loss. This switching circuit will also demonstrate high isolation between the common port and the three open ports, as well as high isolation between each of the selectable ports.
While the design depicted by FIG. 1 is believed to be novel, it has several flaws. Ideally, all four MEMS devices 10-1 through 10-4 should be clustered as close as reasonably possible around a single point 6. In FIG. 1, note that switches 10 have different spacings from end point 6. When the switches 10 are separated by a length of transmission line, as is the case in FIG. 1, that length of transmission line will then serve as a parasitic reactance to some of the ports. For example, in FIG. 1, the length or portion of transmission line designated by the letter “L” appears as an open microstrip stub to ports 1 and 2. This length L of microstrip 6 is referred to as a “stub” in the antenna art and it affects the impedance of the circuit in which it appears. The effect, in this embodiment, is likely to be undesirable. Unfortunately, the second pair of ports 3, 4 likely may not be brought any closer to the first pair 1, 2, because this would cause unwanted coupling between the closely spaced sections of microstrip line that would result. Furthermore, if one wanted to compensate for the parasitic reactance caused by the microstrip stub, one would need to separately tune each of the lines because they do not all see the same reactance. There may not be space on the top side of the circuit to allow a separate tuning element for each of the selectable ports, and still allow room for the DC bias lines and the RF signal lines.